Since the advent of a resist for KrF excimer laser (248 nm), an image forming method called chemical amplification is used as an image forming method for a resist so as to compensate for sensitivity reduction caused by light absorption. For example, the image forming method by positive tone chemical amplification is an image forming method of decomposing an acid generator in the exposed area upon exposure to produce an acid, converting an alkali-insoluble group into an alkali-soluble group by using the generated acid as a reaction catalyst in the baking after exposure (PEB: Post Exposure Bake), and removing the exposed area by alkali development.
Along with miniaturization of a semiconductor device, there is becoming shorter the wavelength of the exposure light source and higher the numerical aperture (higher NA) of the projection lens, and an exposure machine using an ArF excimer laser having a wavelength of 193 nm as a light source has been so far developed. As commonly well known, these features can be expressed by the following formulae:(Resolution)=k1·(λ/NA)(Depth of focus)=±k2·λ/NA2 wherein λ is the wavelength of the exposure light source, NA is the numerical aperture of the projection lens, and k1 and k2 are coefficients related to the process.
A so-called immersion method of filling a high refractive-index liquid (hereinafter sometimes referred to as an “immersion liquid”) between the projection lens and the sample has been conventionally advocated as a technique for raising the resolution.
As for the “effect of immersion”, assuming that NA0=sin θ, the above-described resolution and depth of focus in the immersion can be expressed by the following formulae:(Resolution)=k1·(λ1/n)/NA0 (Depth of focus)=±k2·(λ0/n)/NA02 wherein λ0 is the wavelength of exposure light in air, n is the refractive index of the immersion liquid based on air, and θ is the convergence half-angle of beam.
That is, the effect of immersion is equal to use of an exposure wavelength of 1/n. In other words, in the case of a projection optical system with the same NA, the depth of focus can be made n times larger by the immersion. This is effective for all pattern profiles and can be combined with the super-resolution technology under study at present, such as phase-shift method and modified illumination method.
A double exposure technology or a double patterning technology is being advocated as a technique for more enhancing the resolution. This is to make small k1 in the above-described formula of resolution and is positioned as a resolution-increasing technology.
In conventional pattern formation of an electronic device such as semiconductor device, a mask or reticle pattern in a size of 4 to 5 times larger than the pattern intended to form is reduced and transferred on an exposure target such as wafer by using a reduction projection exposure apparatus.
However, the dimensional miniaturization brings about a problem that in the conventional exposure system, lights irradiated on adjacent patterns interfere with each other to decrease the optical contrast. Therefore, in such technology, it is devised to divide the exposure mask design into two or more designs and synthesize an image by independently exposing these masks. In this double exposure system where the exposure mask design is divided, the image of the design must be again synthesized on an exposure target (wafer) and therefore, the division of the mask design must be devised so that the pattern on the reticle can be faithfully reproduced on the exposure target.
Studies of applying the effect of these double exposure systems to the transfer of a fine image pattern of a semiconductor device are introduced, for example, in Patent Document 1 (JP-A-2006-156422 (the term “JP-A” as used herein means an “unexamined published Japanese patent application”)).
Also, the recent progress of double exposure technology is reported, for example, in Non-Patent Document 1 (SPIE Proc 5754, 1508 (2005)), Non-Patent Document 2 (SPIE Proc 5377, 1315 (2004)), and Non-Patent Document 3 (SPIE Proc 61531K-1 (2006)).
However, in these double exposure systems, the pattern formation needs to be performed in the vicinity of resolution limit of the resist and this incurs a problem that sufficient exposure margin or depth of focus cannot be obtained.
In other words, if a pattern forming process described, for example, in Patent Document 2 (JP-A-2001-109154) where a resist composition containing a resin capable of increasing the polarity upon exposure is coated on a substrate and the resist film is exposed and developed to dissolve the exposed area with an alkali developer, or a pattern forming process described, for example, in Patent Document 3 (JP-A-2003-76019) where a resist composition containing a resin capable of increasing the molecular weight upon exposure is coated on a substrate and the resist film is exposed and developed to dissolve the unexposed area with an alkali developer, is applied to a double exposure process, a sufficiently high resolving performance cannot be obtained.
With respect to the developer for g-line, i-line, KrF, ArF, EB or EUV lithography, an aqueous alkali developer of 2.38 mass % TMAH (tetramethylammonium hydroxide) is being used at present as a general-purpose developer.
Other than the above-described developer, for example, Patent Document 4 (JP-A-2001-215731) describes a developer for developing a resist material containing a copolymer of a styrene-based monomer and an acryl-based monomer and dissolving the exposed portion, where the developer contains an aliphatic linear ether-based solvent or aromatic ether-based solvent and a ketone-based solvent having a carbon number of 5 or more; Patent Document 5 (JP-A-2006-227174) describes a developer for developing a resist material capable of decreasing the molecular weight as a result of breakage of the polymer chain upon irradiation with radiation, thereby dissolving the exposed portion, where the developer has at least two or more acetic acid groups, ketone groups, ether groups or phenyl groups and has a molecular weight of 150 or more; and Patent Document 6 (JP-A-6-194847) describes a developer for developing a resist material mainly composed of a photosensitive polyhydroxy ether resin obtained by the reaction of a polyhydroxy ether resin with a diglycidyl (meth)acrylate, where the developer is an aromatic compound having a carbon number of 6 to 12 or a mixed solvent containing 50 mass % or more of an aromatic compound having a carbon number of 6 to 12.
However, these combinations of the resist composition and the developer mentioned above merely provide a system of forming a pattern by combining a specific resist composition with a high-polarity alkali developer or a developer containing a low-polarity organic solvent.
That is, as shown in FIG. 1, in the case of a positive tone system (a combination of a resist composition and a positive tone developer), a material system of performing the pattern formation by selectively dissolving and removing a region having strong light irradiation intensity out of an optical aerial image (light intensity distribution) is merely provided. On the other hand, as for the combination of a negative tone system (a resist composition and a negative tone developer), a material system of performing the pattern formation by selectively dissolving and removing a region having a weak light irradiation intensity is merely provided.
The term “positive tone developer” as used herein indicates a developer that selectively dissolves and removes the exposed area not lower than a predetermined threshold value shown by a solid line in FIG. 1, and the “negative tone developer” indicates a developer that selectively dissolves and removes the exposed area not higher than the predetermined threshold value. A development step using a positive tone developer is called a positive tone development (sometimes referred to as a positive tone development step), and a development step using a negative tone developer is called a negative tone development (sometimes referred to as a negative tone development step).
Patent Document 7 (JP-A-2000-199953) describes a double development technology as the double patterning technology for improving the resolution. In this case, an image forming method by chemical amplification in general is utilized, and by making use of a property that the polarity of a resin in a resist composition when exposed becomes a high polarity in a region irradiated with a high light intensity and becomes a low polarity in a region irradiated with a low light intensity, positive tone development is performed by dissolving a high exposure region of the resist film with a high-polarity developer, while negative tone development is performed by dissolving a low exposure region with a low-polarity developer. More specifically, the region not lower than an exposure dose of E2 in FIG. 2 is dissolved using an aqueous alkali solution as the positive tone developer, the region not higher than an exposure dose of E1 is dissolved using a specific organic solvent as the negative tone developer, and, as shown in FIG. 2, the region with a medium exposure dose (E2 to E1) is allowed to remain without being developed, whereby an L/S pattern 3 with a pitch half the pitch of the exposure mask 2 pattern is formed on a wafer 4.    Patent Document 1: JP-A-2006-156422    Patent Document 2: JP-A-2001-109154    Patent Document 3: JP-A-2003-76019    Patent Document 4: JP-A-2001-215731    Patent Document 5: JP-A-2006-227174    Patent Document 6: JP-A-6-194847    Patent Document 7: JP-A-2000-199953    Non-Patent Document 1: SPIE Proc 5754, 1508 (2005)    Non-Patent Document 2: SPIE Proc 5377, 1315 (2004)    Non-Patent Document 3: SPIE Proc 61531k-1 (2006)